Field of the Invention
The invention pertains to processes for producing partly-hydrolyzed cellulose by reacting cellulosic materials with acid.
Background of the Invention
The production of partly-hydrolyzed cellulose by hydrolysis of cellulosic materials from sources such as wood pulp and cotton is a well-known process. In this process, sulfuric, hydrochloric or other strong acid or acid mixture is used to partially hydrolyze finely ground cellulose particles to liberate partly-hydrolyzed cellulose (which is also referred to in the literature as cellulose crystallites or nanocrystalline cellulose). The reaction temperature, acid concentration and reaction time can vary somewhat but they are typically in the range of 60 degrees C.+/−10 degrees C., 60 wt %+/−10%, and about 20 to 60 minutes, respectively, in a stirred batch reactor. Another parameter that can be varied is the acid to cellulose weight ratio, which is typically in the range 10 to 20:1. Once the desired reaction time has been reached, the mixture is quenched with a suitable quantity of water or dilute acid, about ten times the initial mixture mass, to cease the reaction. The bulk of the diluted acid is then removed, typically by centrifugation or dialysis, or both. The diluted acid is neutralized and disposed of, or purified (to remove sugars produced during hydrolysis) and reconcentrated for re-use in the process. The disposal or recovery of this large mass of acid represents a very significant capital or operating expense.
The conventional prior art procedure for producing partly-hydrolyzed cellulose uses a relatively high acid to cellulose weight ratio, on the order of approximately 10 to 20:1 as mentioned above. Even at these relatively high ratios, the mixture produced is very viscous and it is difficult to effectively mix or contact the two components together, especially since the ground cellulose has a very low density, such that the two volumes are similar.
The above process steps are described in the prior art literature, for example: (1) U.S. Pat. No. 5,629,055 to Revol et al.; (2) U.S. Pat. No. 5,188,673 to Clausen et al.; (3) U.S. Pat. No. 5,972,118 to Hester et al.; (4) Jean-Francois Revol, Louis Godbout, Xue-Min Dong, Derek G. Gray, Henri Chanzy, and Georg Maret, “Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation,” Liquid Crystals, (1994) Vol. 16, No. 1: 127; and (5) Xue Min Dong, Tsunehisa Kimura, Jean-François Revol, and Derek G. Gray, “Effects of Ionic Strength on the Isotropic-Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites,” Langmuir, (1996) Vol. 12: 2076; (6) Qian Xiang, Y. Y. Lee, Päro Pettersson, Robert W. Torget, “Heterogeneous Aspects of Acid Hydrolysis of α-Cellulose,” Applied Biochemistry and Biotechnology, (2003), Vol. 107: 505-513.
It is important to the prior art hydrolysis reaction that there be a high degree of mixing or contact between the acid and cellulose; otherwise a large variation in cellulose exposure or reaction times can result in a high degree of unreacted cellulose or over-reacted cellulose. This becomes considerably more difficult to achieve at lower acid to cellulose ratios. One reason is that the volume of ground cellulose, at a typical density of about 100 kg/m3, becomes equal to or greater than the volume of acid it is being mixed into, which when sulfuric acid is used, has a density of about 1500 kg/m3. The second reason is that the resulting mixture has a very high viscosity and behaves more like a thick paste than a liquid for some or all of the reaction time.